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Centrifugal fan | Radial fan

What is a centrifugal fan?

A centrifugal fan (also referred to as a radial fan) is a type of fan that draws in air or other gaseous media in the direction of the motor axis (axial) and deflects it through 90°, before blowing it out again radially. An impeller with blades rotates to drives the air movement using aerodynamic forces. The deflection takes place in the fan.

 Top performance in numerous areas of application: Centrifugal fans from ZIEHL-ABEGG

How does a centrifugal fan work?

From the outside, centrifugal fans look similar to drums. The impeller is driven by an electric motor and draws air in through the inlet nozzle in an axial direction, parallel to the rotary axis. The rotation of the impeller deflects the air by 90° and accelerates it outwards. As a result, the discharge direction is radial to the rotary axis.

Centrifugal fans can generate higher levels of pressure or overcome greater pressure losses than axial fans.

Design measures, e.g. number and shape of the blades, impeller geometry, etc., mean that the centrifugal fan can be calculated precisely according to the requirements of specific applications.

How are centrifugal fans constructed?

Centrifugal fans typically consist of a drive motor, the impeller and an inlet nozzle. In some use cases, the fan is surrounded by housing.
Depending on the motor version, its speed can be controlled by connecting a frequency inverter or by means of integrated electronics (EC motors).

Depending on their design, the impellers of centrifugal fans differ in terms of their blade shape and their forward or backward-curvature.

  • Forward-curved impellers require a helical spiral housing, which directs the discharged air in a particular direction. Due to their high output density, forward-curved impellers can achieve the same air handling capacity at a smaller size or lower speed compared to backward-curved impellers. Due to their mechanical design, forward-curved impellers are more likely to operate at lower speeds and are therefore used at smaller volume flows and lower pressure levels. In the case forward-curved centrifugal fans, there is a direct relationship between power consumption and flow rate, which means the operating point can be controlled by means of motor current and speed measurement.
     
  • In the case of backward-curved impellers, the efficiency of centrifugal fans can be significantly influenced by the design of the blades, e.g. their fluting. They are often welded, which allows higher mechanical loads due to higher speeds and thus higher pressures compared to forward-curved impellers. Backward-curved centrifugal fans can be operated as free-running fans with or without housing. Free-running fans often feature a modular design as a plug fan or ventilation unit to make installation easier for the user (plug & play). In the case of backward-curved centrifugal fans, air flow is often measured via a differential pressure measurement at the nozzle. Since there is no direct relationship between power consumption and flow rate, control via motor current is not possible.

Additional components can be used to optimise centrifugal fans for the various applications and installation situations, e.g.:

  • Inlet guide grilles optimise inflow in confined installation conditions
  • Intake guard grilles prevent intervention into the rotating impeller
  • Vibration dampers prevent the transmission of vibrations

Electric motor (drive motor)

Plastic ring

Centrifugal impeller

Inlet nozzle

Inlet guide grille

 ZAflow from ZIEHL-ABEGG optimises the inflow to reduce noise development

At a glance: Forward & backward curved centrifugal fans

Forward curved Backward curved
Housing Mandatory Not mandatory
Volume flow Smaller volume flows Larger volume flows
Pressure Lower pressure levels Higher pressure levels possible
Speed range Quite low Higher speeds possible
Control Possible via speed & motor current Cannot be controlled via motor current

How are centrifugal fans installed?

Gehäuseventilatoren
Freilaufende Ventilatoren

When installing centrifugal fans, a distinction is drawn between two types of installation situation:

  • In case of fans installed inside housing, air intake can take place on one or two sides. A pipe connection can be connected on the suction or pressure side to direct the air into a duct system or to integrate the fan into a pipeline system.
     
  • In the case of free-running fans, air is sucked in freely and discharged into the measuring line of a wind tunnel (also called a plenum), where pressure is built up. Free-running fans are often used in central air-conditioning units, for example. Ideally, vibration dampers are also used if the fan is mounted on a unit base. For wall mounting, they are attached to the air conditioner via the suction-side plate. This ensures easy cleaning of the device, as the device base is freely accessible.

What are the advantages of centrifugal fans?

  • High pressure build-up: Due to their design, centrifugal fans achieve higher pressures than axial fans, while still delivering high air volumes.
  • No typical stall point: Centrifugal fans can be used across their complete characteristic curve without overloading the motor.
  • High level of efficiency: Centrifugal fans offer a higher level of efficiency compared to axial fans.
  • Low noise levels: Impellers with forward-curving blades have lower efficiency levels but are quieter than backward-curved impellers.
  • Easy installation of free-running fans.
  • Modular design possible: Multiple fans can be mounted to form a fan wall. As a result, inefficient single fans can be replaced by energy-saving fans in retrofit projects, for instance.

​ Maximumefficiency, permanently low operatingcosts: The RETROFIT programme from ZIEHL-ABEGG

Where are centrifugal fans used?

Centrifugal fans are used to meet higher pressure requirements, for example when the air needs to be guided through long pipelines, filters or air treatment components. Typical areas of application include:

What makes centrifugal fans quiet?

Flow-related phenomena are often the reason why dominant noises are generated while a centrifugal fan is running. The fan accelerates the air, sets it in motion and thus provides an air flow. The air is deflected on the fan's blades. The more powerfully this is deflected, the more pressure the fan can provide. Swirling or turbulence of the air occurs at various points on the fan (leading and outlet edge, head gap or surface). In addition to the operating point, their appearance and characteristics are also significantly dependent on the installation situation.

However, an optimum blade geometry design can significantly improve the various flow situations and thus reduce sound radiation. When it comes to developing optimum fan aerodynamics, biomimetics can provide innovative approaches to improving efficiency, noise development or operating behaviour.

Examples:

  • Specially designed blade edges: Corrugated leading edge, spiked trailing edge...
  • Blade surface structures
  • Blade fluting
  • etc.
     

“Centrifugal fan” or “Radial fan”?

In addition to the typical term "centrifugal fan", the term "radial fan" is also often used in day-to-day language, though both terms ultimately mean the same thing. In practice, centrifugal fans in housing are also referred to as “radial blowers”.

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